1. Field of the Invention
This invention relates to a method and an apparatus for augmenting the fuel injection amount at the time of hot restart of an internal combustion engine.
2. Description of the Prior Art
Normally, the fuel supply to an internal combustion engine is increased on an engine start by prolonging the valve open period of a fuel injector, thereby to ensure a good engine start. Since a colder engine requires a greater amount of fuel for starting, the fuel is supplied in an increased amount at a lower engine temperature. However, if the fuel in the vicinity of a fuel injector is in a heated state as is the case when restarting an engine immediately after a short stop, vapors occur in the fuel to be supplied by the fuel injector, so that the fuel supply becomes less than a required amount even if the fuel injector is operated to increase the fuel supply by opening its valve for longer time periods according to the engine temperature. As a consequence, an air-fuel mixture leaner than a required air-fuel ratio is supplied to the engine cylinder, making restart of the engine difficult.
Therefore, an attempt has been made to improve the engine restart quality by a control in which, at the time of a hot restart, the fuel pressure to be applied to the fuel injector is elevated for a certain time length while prolonging the fuel injection time, to compensate the air-fuel ratio in such a manner as to approach the required value. However, for enriching the air-fuel mixture on hot restarts, it has been the conventional practice to increase the amount of fuel injection when the intake air temperature exceeds a predetermined value, failing to control the fuel supply finely with sensitivity to the degree of fuel evaporation in the fuel duct.
For augmenting the fuel injection, there are two methods, namely, a method of prolonging the valve open period of the fuel injector and a method of raising the fuel pressure to be supplied to a fuel injector. The first-mentioned method, which controls the fuel injection time, corrects a basic injection period according a fuel increment coefficient at the start of an engine to prolong the valve open period of a fuel injector.
On the other hand, the method of augmenting the fuel injection on hot restart by boosting the fuel pressure to be supplied to a fuel injector utilizes a pressure regulator which controls the fuel pressure. Normally, intake manifold vacuum is led into a diaphragm chamber of the pressure regulator, controlling the fuel pressure in such a manner as to maintain a constant pressure differential between the fuel pressure and the intake manifold vacuum. On hot restart, when the cooling water temperature of the engine or the intake air temperature exceeds a predetermined value, the control pressure which is admitted into the diaphragm chamber of the pressure regulator is switched to the atmospheric pressure by means of a vacuum switching valve (VSV), thereby raising the fuel pressure for a predetermined time length on a hot restart in a degree corresponding to the pressure difference of the intake manifold vacuum to preclude overlean air-fuel ratios resulting from the fuel evaporation.
Upon lapse of a predetermined time after a hot restart, namely, at a time point when the vapors are considered to have mostly vanished from the fuel duct, the atmospheric pressure which has been admitted into the pressure regulator is switched to the intake manifold vacuum, thereby restoring the normal operating condition of the fuel pressure for the fuel injector.
As a result of researches in this regard, the present inventor has found that, when restarting an engine after a high-load operation, the necessary fuel augmentation is governed by the engine rest time (dead soak time) and the ambient temperature.
Nevertheless, according to the conventional method which increases the fuel supply by elevating the fuel pressure, the control of fuel pressure is effected when the temperature of intake air or cooling water of the engine exceeds a predetermined value, so that it is capable of only a simple control of the fuel pressure based on a present value in spite of variations in other parameters such as dedad soak time, atmospheric temperature and the like. Therefore, there often arises a situation that the pressure in the diaphragm chamber of the pressure regulator is raised to a high level when it is unnecessary or not raised to a high level when it is necessary, supplying an overrich or overlean air-fuel mixture by failure of controlling the increase of fuel supply appropriately on a restart of the engine and thus resulting in insufficient improvement of the engine starting quality.